1887.] on Bridging the Firth of Forth. 147 



a structure of any size or weight. The great board of 300 square 

 feet area on the same occasion indicated only 35 lbs. per square foot, 

 and I doubt much if the pressure would have averaged more than 

 20 lbs. on so large a surface as the bridge. 



Manufacture. — The bent plates required for the tubes of the 

 Forth Bridge would, if placed end to end, stretch 42 miles. Special 

 plant had to be devised for preparing these plates. Long furnaces, 

 heated in some instances by gas producers, and in others by coal, first 

 heated the plates, which were then hauled between the dies of an 

 800 ton hydraulic press, and bent to the proper radius. When cool 

 the edges were planed all round, and the plates built up into the 

 form of a tube in the drilling yard. Here they were dealt with by 

 eight great travelling machines, having ten traversing drills radiating 

 to the centre of the tube, and drilling through as much as 4 in. of 

 solid steel in places. 



The tension members and lattice girders generally are of angle 

 bars, sawn to length when cold, and of plates planed all round. 

 Multiple drills tear through immense thickness of steel at an 

 astonishing rate. The larger machines have ten drills, which, going 

 as they do, day and night, at 180 revolutions per minute, perform 

 work equivalent to boring an inch hole through 280 ft. thickness of 

 solid steel every twenty-four hours. 



Erection. — Facility of erection is one of the most important 

 desiderata in the case of the Forth Bridge. Owing to 200 ft. depth 

 of water, scaffolding is impossible, and the bridge has to constitute 

 its own scaffolding. The principle of erection adopted was therefore 

 to build first the portion of the superstructure over the main j)iers, 

 the great steel towers, as they may be called, although really parts 

 of the cantilever, and to add successive bays of the cantilever, right 

 and left of these towers, and therefore balancing each other, until 

 the whole is complete. This being the general principle a great 

 deal yet remained to be done in settling the details, ^yhat was 

 finally settled, and is now in progress, is as follows : — 



After the skewbacks, horizontal tubes, and a certain length of the 

 verticals as high as steam cranes could conveniently reach were 

 built, a lifting stage was erected. This consisted of two platforms, 

 one on either side of the bridge, and four hydraulic lifting rams, one 

 in each 12 ft. tube. To carry these rams cross girders were fitted in 

 the tubes capable of being raised so as to support the rams and 

 platform as erection proceeded, and steel pins were slipped in to 

 hold the cross girders. Travelling cranes are placed on the plat- 

 forms, and these cranes, with the men working aloft, are of course 

 raised with the platforms when hydraulic pressure is let into the 

 rams. The mode of procedure is to raise the platform 1 ft. and 

 slip in the steel pins to carry the load whilst the rams are getting 

 ready to make another stroke of 1 ft. "When a 16 ft. lift has been so 

 made, which is a matter of a few hours, a pause of some two or three 

 days occurs to allow the riveting to be completed. The advance at 



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